The study looked at different forests in Bolivia, the USA, and Spain to understand why tree diversity varies across regions. They found that the environment plays a bigger role in diversity than how close trees are to each other. In more diverse and productive areas, where there are more rare species, the environment has a bigger impact on diversity. Having more types of trees in an area doesn't always mean there will be more differences between them. The differences in tree diversity are mostly due to how the tree communities are put together, rather than just where they are located.